1. Field of the Invention
The presently disclosed invention is directed to the use of switchable films such as electrochromic materials in automotive applications and, more particularly, the serviceability of SPD and PDLC films in automotive window glazing.
2. Discussion of the Prior Art
There are a number of technologies for changing and controlling the color or transmittance of a glazing panel. Electrochromics is one of the technologies that are sometimes used in automotive applications. Switchable films based on other technologies may also be used. Electrochromics concerns a reversible reaction (chemical or physical) that is induced by the application of an electrical current or electrical potential across an emulsion or film. In certain electrochromic devices, a suspended particle device (“SPD”) emulsion or a polymer dispersed liquid crystal (“PDLC”) film is confined between two transparent substrate plates. In the case of SPD films, the substrate plates can be made of polyethylene terephthalate (“PET”) that are coated with transparent, electrically conductive coatings. When an electrical voltage is applied across the film such as, for example, by applying an ac voltage to the electrically conductive coating on the PET, a change in the color or light transmittance of the emulsion occurs.
Such suspended particle device (“SPD”) film or polymer dispersed liquid crystal (“PDLC”) film is sometimes referred to as “smart glass” technology. As used in windows in automotive vehicles and similar applications, it is incorporated in “switchable glazing” that allows for the control of light transmittance by controlling an electrical voltage that is applied across the film.
A problem in the prior art has been that such switchable glazings have not had adequate “serviceability.” That is, prior art switchable glazings have tended to degrade over time due to exposure to ultraviolet (“UV”) light, high temperatures, and chemical reactions with substances in the ambient environment or substances used in the glazing construction. Automotive applications expose switchable glazings to all of those conditions. Over time, such degradation compromises the capacity of switchable glazings to switch between a relatively translucent or opaque state and a relatively transparent state.
Another problem with the use of electrochromic films in switchable glazing has been optical distortion that results from the inclusion of the electrochromic film, such as SPD switchable film, in the glazing. For example, a typical thickness of SPD switchable film is on the order of approximately 0.38 mm. Exposure of the edges of the SPD switchable film to ambient conditions can result in degradation of the SPD switchable film due to, for example, chemical attack. To protect the edges of the SPD switchable film, the SPD switchable film is sometimes centered in the laminate with a boarder of glass and interlayers that extend beyond the perimeter of the SPD switchable film. When the assembly of layers is laminated in an autoclave, the layer assembly in the margin surrounding the perimeter of the SPD switchable film is thinner than the area that includes the SPD switchable film by as much as the thickness of the SPD film. This difference in thickness resulted in curvature of the glass in the areas adjacent to the perimeter of the SPD film that has caused an unacceptable degree of optical distortion. In addition, where the glass and interlayer near the perimeter of the SPD switchable film did not successfully bond, included voids in the region adjacent to the perimeter of the SPD switchable film could sometimes result, or, if the glazing is thereafter exposed to sufficiently high temperatures, resilience of the plies could flex the plies to an unstressed condition and cause the interlayer to draw in so as to create a scant in the glazing.
Switchable films also have presented other disadvantages and difficulties that tend to limit and compromise their use in automotive applications. Accordingly, there was a need in the prior art for a switchable glazing construction that was serviceable and effective for automotive applications.